3 Answers
3

This kind of flash diffuser produces a "bare-bulb" effect. It's not like a softbox or umbrella, which works by effectively making a larger light source. Instead, it makes the light from your flash less focused, so it's diffused by bouncing off walls and other objects.

Normally, a hotshoe flash works like a spotlight — it focuses its output in a cone. That's good for efficiency, but makes the light rather uni-directional and harsh. A bare bulb, on the other hand, scatters light in every direction. From the point of view of a subject, it's still one source, but the scattered light also bounces around the room, providing softer fill light.

Many people try to use this type of push-on diffuser to improve the appearance of flash when shooting in large spaces or outdoors. This will lead to disappointment. The little bit of plastic doesn't do much but cut light output in this case.

The Sto-Fen Omnibounce is one of the more popular brand-name pieces of push-on plastic, and they have a diagram of the effect on their website. Note the bouncing arrows — that's critical, even though they don't really explain that every well.

Of course, a real bare-bulb (also called "bare tube") flash just doesn't have a focusing reflector in the first place. This is a lot more effective than having the reflector and sticking a little white plastic box on top. I'm not aware of any hotshoe units made to work this way (short of somewhat-dangerous D.I.Y.), but many studio lights (like this) are made to easily support this configuration. Notice the 45° angle suggested in the Sto-Fen diagram — that's basically a compromise because the small push-on thing isn't big enough to really scatter light everywhere. Devices like Gary Fong's "Lightsphere" are meant to provide a bigger "bulb emulator".

I think the push-on diffusers for hotshoe flashes are mostly a gimmick, but they do have some use. When you're bouncing from the ceiling, sometimes light is still too directional and you get unwanted downward shadows. Since we normally expect light like that, that's better than shadows on the wall and otherwise sideways — but using this device will scatter some more light around to bounce in more directions (including directly, of course).

My experience is almost the opposite of mattdm's - I only ever use my flash with the diffuser as I find the resulting lighting is far too hard. I can't counter his physics, just speak from personal experience...
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Danny EdmundsJan 28 '12 at 17:55

Hot-shoe, no, but flashes designed to be carried around attached to the camera, yes. I guess the poster child now is Quantum, but the old Sunpak 622 did bare-bulb as well. I'm surprised, really, that there aren't more of them around, especially from folks like Canon and Nikon; they were staples in the wedding photo business (Monte Zucker built an entire career around bare bulb) and you don't need nearly the power with a small-format DSLR that we did with medium-format film (ISOs can be much higher and apertures wider for the "same" shot).
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user2719Jan 28 '12 at 20:39

@Danny: yeah, it should have a pretty decent softening effect when used where you can get bounced light. In other situations, the softening may simply be due to the reduced flash power.
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mattdmJan 28 '12 at 20:57

P.S.: my comments here are also from experience, not just physics in the abstract. :)
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mattdmJan 28 '12 at 23:36

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@mattdm - So to use this correctly you need to be indoors in a fairly small room (or at least close to the walls) and aim the flash horizontally at the subject and vertically 45deg up, the resulting light is less directional than bouncing the light without a diffuser and so has less/softer shadows - is this correct?
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NirJan 29 '12 at 8:07

As has been described above, the omni-diffusers do not expand the volume of the light source, rather they only expand the cone of light beyond the quite directional cone a bare flash provides. I often see photographers in clubs or bars using these and the results speak for themselves. Often the light will harshly falloff down past the person's face and result in a largely top exposed photo.

I've come to enjoy using a bounce card attached to my flash. This is my own setup using a 60D and a Canon 430EX II. I have the flash angled around 75 degrees from horizontal and the bounce card is angled just enough so that it cover the direct line of sight of the flash head (not the whole cone of light, mind you).

What this does is creates essentially a key light and fill light in one go. A portion of the light will travel past the bounce card and interact with the ceiling, adding some definition, while the bounce card will pickup the majority of it and fill in the face(s). Not only that, it is also high enough to add it's own angle of definition. This is not the only solution, but it works very well for me. I've attached a photo that I've taken with this setup.

Setup:

Light path (roughly):

Result:

You can see that it fills the entire photo with light, but using a flattering angle and softer edge on the shadows.

You won't get "modified" light from an unmodified light. That is, pushing a diffuser over the front of a speed light isn't an instant softbox. Neither is Gary Fong's Lightsphere, although you see them all over the place. You have three directions to go here:

Ignore everything, love your flash the way it is, and live with the harsh lighting

Add a diffuser and bounce the flash. It's not really useful to keep the flash pointed at your victim, er, subject and just expect a piece of plastic to give you magic lighting. Better than nothing, but if you can bounce the light, it all of a sudden becomes a much bigger light source and the results can be pretty good

Invest in some real mods. This doesn't sound like where you are going, as they are only intended for off-camera work, and can get pricey pretty fast.

In terms of the on-camera/off-camera question, it's conventional wisdom that direct on-camera flash is about the worst kind of light if it is your main light. In your case, it seems you are using the flash as the main light, so the shadows will be unflattering and the light falloff quite noticeable behind the subject. If you move the flash off camera, at least the shadows will help define the shape of the subject. Above and to the side would be a good place to start. You very much have to visualize the path the light will take from your flash to the subject to make this work and the diffuser may or may not work in your favor here.

The point of the diffuser (and the Lightsphere) is that the light will bounce, giving you both a directional main light and diffuse fill from a single source. And yes, it works (although as mattdm noted, you do need to be in an environment where reflection is available). Don't knock it until you've tried it -- "bare bulb" is great lighting once you learn how to use it.
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user2719Jan 29 '12 at 2:16

The point I was trying to make is that you don't get automatic soft light. Most photojournalists carry these diffusers for use "in a pinch." They're just fine when there are no other options. But "bare bulb" is a hard type of light to control, so its general applicability is a good deal lower than a softbox or even bounce flash.
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Steve RossJan 29 '12 at 6:30

Moving the flash off-camera and investing in some real mods is where I'm going - it's just going to take me a while to get there because this is an hobby for me and I don't have the budget (but I did have great success with some DIY light mods) -- I actually expected the plastic diffuser to be totally worthless and just one of those things camera stores make profit on (like a cheap UV filter) and I'm actually pleasantly surprised it has some use at all - and I'm going to test it the next time I get a victim for a portrait session.
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NirJan 29 '12 at 8:21

Bare bulb is a hell of a lot easier to control than, say, lugging around stands and softboxes/umbrellas everywhere you go and making everything a static "set". Like anything, you need to learn how to use it -- but it's not a tool anybody who wants to do location/event shooting should be without. And no, the diffuser isn't for "in a pinch", it's for "setting up a small version of my studio is really inappropriate for most pictures outside of my studio". Soft light all of the time is a cop-out; you get generic, safe pictures rather than good ones.
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user2719Jan 29 '12 at 11:46

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I should have said "shaped" instead of "modified". There's nothing wrong with these -- they simply don't work as a silver bullet. I don't lug a bunch of stands and mods around either. But I might bring a LumiQuest softbox (goo.gl/V2fUh) or one of the Rogue Flashbenders (review here: goo.gl/7kxmd). I find them more flexible and the results more pleasing. If I hold the flash off camera or have someone else hold it, so much the better. There are times when my only choice is the diffuser. But it's not my first choice if I have time.
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Steve RossJan 29 '12 at 17:54